High-Resolution Laser Spectroscopy
high-resolution spectroscopy requires the narrow-bandwidth excitation sources that are only achievable with lasers. Studies in the visible spectral region typically use a tunable dye laser and studies in the near-ultraviolet and near-infrared are becoming more common as frequency-doubling and wave-mixing methods improve. Near-infrared diode lasers are also used for high-resolution vibrational spectroscopy.
High-resolution studies require cooling of the molecules to remove spectral congestion and to reduce the Doppler width of the transitions. Gas-phase studies use free-jet expansions or molecular beams to cool molecules to very low temperatures. Large molecules are commonly dissolved in a suitable solvent and cooled to cryogenic temperatures to form a glass or crystalline matrix.
Preparing molecules in a collimated beam greatly reduces the distribution of velocities perpendicular to the beam direction. The beam is produced by effusion through a pinhole and a skimmer, and the laser direction must be perpendicular to the direction of the beam. Since the velocity component parallel to the laser is greatly reduced the Doppler width is likewise reduced.
Science Hypermedia Home Page
Copyright © 1996 by Brian M. Tissue